1. Field of the Invention
The present invention relates to a crystal oscillator and, in particular, to a crystal oscillator that holds a crystal element by using a eutectic alloy.
2. Description of Related Art
A crystal oscillator is known as a frequency control element, and is used as an oscillation generator in an oscillator of a communications device, by way of example. In one type of this crystal oscillator is a crystal oscillator for high stability use wherein a eutectic alloy is used to hold a crystal element that is sealed and enclosed within a metal container.
FIG. 3 is illustrative of an example of a prior-art crystal oscillator, with FIG. 3A being a longitudinal section therethrough, FIG. 3B being a plan view of essential components thereof, and FIG. 3C being a partial expanded longitudinal section therethrough.
As shown in FIG. 3, this crystal oscillator is provided with a metal base 1, supporters 2, a crystal element 3, and a metal cover 4. The metal base 1 has a flange 1b around the outer periphery and at least a pair of sealed terminals 5 pass through the main body of the metal base 1. In this case, two pairs (a total of four) of the sealed terminals 5 pass through the metal base 1. A lead wire of each of the sealed terminals 5 protrudes on one main surface side of the crystal element 3 and is led out to the other main surface thereof. The sealed terminals 5 are positioned on straight lines that mutually cross in concentric circles.
Each supporter 2 is formed of a metal such as nickel (Ni) and is a flat plate of a substantially L-shaped section. The leading end of one of the sealed terminals (lead wires) 5 that protrudes towards one main surface of the base 1 is welded by a laser to the horizontal portion of the L-shape thereof. In this case, the L-shaped horizontal portion extends towards the center and the flat surface of the perpendicular portion thereof faces the crystal element 3.
The crystal element 3 is an SC-cut or AT-cut crystal, of a circular shape as shown in FIG. 3B. This type of crystal oscillator has a main surface electrode 6 provided on each of the two main surfaces of the crystal element 3 and an end surface electrode 7 that is formed to extend between the two main surfaces. The main surface electrodes 6 have excitation (base) electrodes 6a that face each other in mutually opposite directions on the two main surfaces and extraction electrodes 6b that extend from each excitation electrode 6a on opposite sides in mutually opposite directions. The excitation electrodes 6a and extraction electrodes 6b are formed by deposition on the two main surfaces of the crystal element 3. A first underlayer 8a formed of a material such as chrome (Cr) is disposed on one main surface of the crystal element 3, and a first surface layer 9a of gold (of Au) is superimposed on the first underlayer 8a, as shown in FIG. 3C.
The end surface electrodes 7 are formed as one pair of end portions A and A′ on the crystal element 3 extending from the extraction electrodes 6b, as shown in FIG. 3B, and another pair of end portions B and B′ perpendicular thereto. Each of these end surface electrodes 7 is formed of a nickel-chrome (of NiCr) alloy and is shaped to extend between the two main surfaces of the crystal element 3. Note that each end surface electrode 7 that extends between the main surfaces is formed so that the end portions A and A′ are superimposed on the extraction electrodes 6b whereas the other two end portions B and B′ are formed directly on the crystal element 3.
A eutectic alloy 10 formed of a gold-germanium (of AuGe) alloy is interposed between a side surface of each of the two end portions A and A′ and the other two end portions B and B′ of the crystal element 3 and the vertical surface of the corresponding supporter 2, as shown in FIG. 3C. The eutectic alloy 10 could be fixed to the vertical surface of the supporter 2 by being melted, by way of example. The side surfaces of the crystal element 3 are abutted against the four supporters 2 by means of a jig or the like, and the assembly is heated to approximately 400° C. This causes the eutectic alloy 10 to melt, enabling bonding of the end surfaces, including the main surfaces of the crystal element 3, to the supporters 2 and thus holding the crystal element 3 horizontally above the metal base 1.
The metal cover 4 is subsequently bonded to the metal base 1 by a method such as cold welding, to seal and enclose the crystal element 3 within the metal cover 4.
In the thus-configured crystal oscillator, the crystal element 3 can be bonded to the supporter 2 by the eutectic alloy 10 without having to use materials such as an electrically conductive adhesive, to obtain favorable oscillation characteristics without generating organic gases. It can therefore be applied to a crystal oscillator for high-stability use.
The end surface electrodes 7 are configured to NiCr and are superimposed over the extraction electrodes 6b of the main surface electrodes 6, made of Au. Thus when the eutectic alloy 10 is melted, the gold (of Au) of the main surface electrode 6 is absorbed by the diffusion of the eutectic alloy 10 and the NiCr, to prevent the phenomenon known as gold corrosion. If only the end surface of the end surface electrode 7 made of NiCr were superimposed over the extraction electrode 6b of the main surface electrode 6, the eutectic alloy 10 would flow over the main surface electrode 6 (of Au) and induce the gold corrosion phenomenon. As a result, the main surface electrode 6 (of Au) will peel off from the crystal element 3, which will cause conductivity failures (refer to Japanese Patent Laid-Open Publication No. 2003-332876).
In addition, since the end surface electrodes 7 of the prior-art crystal oscillator of the above configuration is formed only of NiCr, the adhesive strength thereof with the eutectic alloy 10 is weak, leading to a problem concerning low bond strength. An impact or the like could cause the crystal element 3 to separate from the supporters 2, which would cause a deterioration in shock resistance.
For that reason, the end surface electrode 7 of NiCr of the prior-art crystal oscillator has a configuration such that a second surface layer 9b formed of Au is superimposed on a second underlayer 8b formed of NiCr, as shown in FIG. 4. It should be noted that the second underlayer 8b (of NiCr) protrudes further towards the center on the main surface of the crystal element 3 than the second surface layer 9b (of Au). However, similar gold corrosion of the second surface layer 9b (of Au) can occur in this case, due to the eutectic alloy 10 (of AuGe), so that the bond strength with the second underlayer 8b (of NiCr) becomes weaker, which causes problems concerning unexpected peeling from the crystal element 3.
The present invention has the objective of providing a crystal oscillator in which this peeling is prevented and the bond strength is increased by bonding with a eutectic alloy.